In reply to L'Eeyore:
Please do post your conclusions, I am happy to be corrected but am fairly sure I am right.
Essentially, if the problem can be fixed by issuing an excess fare, the ticket office can fix it. That would be things like change of class, change of route[1], wanting to travel further etc, and possibly, though it is debatable, changing a Day Return into a period return provided it is done before expiry (some ticket offices will do this, some won't). If it requires a refund and reissue, as a date change on the outward does in most cases[2] they can't.
[1] Geographic route, e.g. "not London" to "via London". Not a lot of people know, but you can do out via one route and back via the other for half the difference between the cheaper and more expensive one - only a ticket office can do that. However you can't change a "Virgin Trains Only" ticket to one without that restriction, for instance - that requires a refund and reissue.
[2] An Anytime Single is valid 2 days anyway, no need to change it to travel on day 2. Same with the outward portion of an Anytime Return which is valid 5 days.
Post edited at 23:18